Grass as part of fat-free diet (4)
I believe the answer is:
reed
'grass' is the definition.
(I've seen this before)
'part of fat-free diet' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'REED' is hidden within 'fat free diet'.
'as' is the link.
(Other definitions for reed that I've seen before include "Tall grass plant that grows in water" , "It grows in water or marshy land" , "Tall woody grass" , "One of two in a 25 [OBOE]" , "Marsh-growing grass" .)